The genome of organisms as varied as bacteria, corn and fruit flies are known to harbor discrete autonomous sequences of DNA (transposable elements) which are capable of transposition to new sites within the genome (Nevers and Saedler, 1977; Calos and Miller, 1980; Starlinger, 1980; Kleckner, 1981: Green, 1980). Although much is known about their genetic and biochemical properties, little is known about their evolutionary origins. I present in this proposal a project aimed at testing the hypothesis that these elements might have evolved by conferring their host genome a selective advantage in the form of an increased mutation rate. I intend to examine this possibility by investigating competition between strains of Escherichia coli with and without specific transposable elements (Tn5, Tn10, and bacteriophages lambda and P1) in chemostat cultures. Transposition of these elements within the chemostat populations will be monitored via Southern blot hybridizations (Southern, 1975). The sites of new transposition will be analyzed genetically and mapped, with the final objective being to ascertain whether the new transpositions occur randomly or whether they are correlated with loci that confer a selective advantage in the chemostats. In preliminary experiments with Tn10, I have been able to detect transpositions and the results suggest that Tn10 does confer a selective advantage by creating beneficial mutations and allowing its host genome to evolve faster. The demonstration that transposable elements have evolved for the specific role of generating genetic variability would be significant. The phenomena of mutations, genetic rearrangements, or gene regulation are fundamental not only to evolution but also to other areas of biology. For example, many processes of both biological and medical interest (genetic disorders, acquisition or loss of virulence by pathogenic organisms, senescence, genetic or physiological adaptations) could all be directly related to an organisms's ability or failure to regulate its genes.